# Magnet repulsive force -- how long does it last?

cel123456
As we know the magnet will stop to repel each other after some time, is there any formulae to calculate when it will stop? From common sense, how long magnet will stop repel each other? 1years?

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Gold Member
Two magnets repel and repel force which depends on distance keeps the same forever. Motion of magnets would die due to other factors e.g., friction.

cel123456
Two magnets repel and repel force which depends on distance keeps the same forever. Motion of magnets would die due to other factors e.g., friction.
Normally how long this repel process will stop?

Staff Emeritus
Normally how long this repel process will stop?

The repulsive force will not stop until the magnets are destroyed/demagnetized (for a permanent magnet) or until the electrical current stops (for an electromagnet).

Gold Member
Normally how long this repel process will stop?
The magnetic force behaves like the gravitational force. The force of attraction of a mass to the Earth (its weight) stays the same as long as other things don't change. Two permanent magnets will experience the same force between them until 'something' changes. They don't 'get tired', if that's what you are suggesting.

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Gold Member
Normally how long this repel process will stop?
Damped oscillation as lectured here might be of your interest.

Gold Member
Damped oscillation as lectured here might be of your interest.

Not really in this discussion. The Energy in the Oscillation will decay but the force pulling the pendulum bob towards Earth at the equilibrium position won't.

rude man
Gold Member
common sense, how long magnet will stop repel each other?
See Paleomagnetism
The oldest rocks on the ocean floor are 200 mya – very young when compared with the oldest continental rocks, which date from 3.8 billion years ago. In order to collect paleomagnetic data dating beyond 200 mya, scientists turn to magnetite-bearing samples on land to reconstruct the Earth's ancient field orientation.

sophiecentaur
Gold Member
{snip}Two permanent magnets will experience the same force between them until 'something' changes. They don't 'get tired', if that's what you are suggesting.
My late wife usually deferred to my EM knowledge except for animism and 'tired magnet' theory. One day a garnet decorated refrigerator magnet popped off the 'fridge and rolled across the kitchen floor.

"Aha", she exclaimed, "It got tired and fell off!", validating her theory.

I countered that only the glue holding the garnet had failed. The flat magnet was still firmly attached to the refrigerator. We literally had to pry the magnet loose when we sold the fridge.

nsaspook and sophiecentaur
Gold Member
The Energy in the Oscillation will decay but the force pulling the pendulum bob towards Earth at the equilibrium position won't.
I see your point. @cel123456 do you share it with @sophiecentaur?
Say your concern comes from sustainability of magnetic force itself not motion around the equillibrium, because loop current generates magnetic force, the loop current is lost if power to the circuit is lost. Superconducting magnet does not need power supply to keep current but it is fragile and tends to do phase transition to normal mode. Loop current of an electron spin is as stable as its charge but order of collective electron spins which make permanent magnet is disturbed by heat or magnetic field coming from environment. All these factors and more would make generated magnetic force unstable.

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Not really in this discussion. The Energy in the Oscillation will decay but the force pulling the pendulum bob towards Earth at the equilibrium position won't.

Pedantic point, but couldn't help myself: the force on the pendulum bob at the equilibrium position is by definition zero, but I guess you mean it's the sum of two oppositely directed forces (weight & magnetic) whose respective [equal] magnitudes at the [fixed] position of equilibrium are the same each time the bob passes through that point.

Gold Member
Haha if you are going to be pedantic then explain, if the force goes to zero, why the bob doesn’t just float away. It’s the RESTORING force that goes to zero. I wrote what I wrote with some care.

Haha if you are going to be pedantic then explain, if the force goes to zero, why the bob doesn’t just float away. It’s the RESTORING force that goes to zero. I wrote what I wrote with some care.

Unfortunately here you are being imprecise [e.g. you just say "force", without specifying if you mean a single force or the resultant], so it's hard to tell if what you wrote is correct or incorrect.

The hovering bar magnet is, at any moment, subject to a downward weight force and an upward magnetic force. At the equilibrium position, the weight exactly balances the magnetic force. Displacing the hovering magnet slightly from its equilibrium position results in the magnetic force either slightly increasing or slightly decreasing, so that in either case the resultant force (weight + magnetic) points toward the equilibrium position. [i.e. this resultant force is what you called the restoring force]

Basically I'm saying your original statement is correct only if by "force" you mean the weight only and not the resultant, and I suspect this is what you actually mean.

Digcoal
Gold Member
the force pulling the pendulum bob towards Earth at the equilibrium position won't.
Basically I'm saying your original statement is correct only if by "force" you mean the weight only and not the resultant, and I suspect this is what you actually mean.

Okay, good! I just thought the original wording was ambiguous and wanted to eliminate any possible doubt for anyone who stumbles across this thread.

Digcoal
Gold Member
The same force pulls the bob towards the Earth at all times; its weight. If I had meant to say 'restoring force' then I would have said it'. If you were really fussy, then you could say that, as the height of the bob above ground does actually vary and that the field above the ground is not strictly uniform but I doubt that the model in your head would include that factor.
Imo you were trying to play the pedantry game (great fun, of course) and chose the wrong playing field.

nsaspook and rude man
Imo you were trying to play the pedantry game (great fun, of course) and chose the wrong playing field.

This is a natural feature of a written-format scientific discussion forum! If there is something vaguely ambiguous then it's good practice to ask for clarification, especially with things like in this thread where it's easy to make conceptual mistakes.

Digcoal and sophiecentaur
Homework Helper
Gold Member
As we know the magnet will stop to repel each other after some time, is there any formulae to calculate when it will stop? From common sense, how long magnet will stop repel each other? 1years?
Assuming you are thinking of permanent magnets, the answer is that the magnet holds electrons spinning about their own axes. If the spin axes are aligned
the electrons produce "amperian currents" along the magnet's surface which by themselves never reduce in intensity. They effectively see zero resistance.
(A much lesser amount of magnetic field is also produced by the electrons' motion about the nucleus.)

But the axes can be partially misaligned due to temperature, mechanical impact, etc. and then the electron spins don't add so much, producing lower net current and therefore magnetic field.

So, bottom line, if the permanent magnet is kept at low temeperature and not subject to shock etc. then the magnet theoretically almost never loses magnetic strength.

weirdoguy
electrons spinning about their own axes.

Um, how?

Homework Helper
Gold Member
cf. Fermi-Dirac statistics.
Also C. Kittel ch. 12, Ferromagnetism and Antiferromagnetism

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Gold Member
Um, how?
Electrons have 'spin' and they are charged. That causes them to behave like small electromagnets with a Moment Dipole Moment. But the 'current' in these electromagnets cannot decay. That's a consequence of Quantum Mechanics.
In most materials, most of the time, the total of electrons with one spin equals the total with the other spin so most materials cannot be permanently magnetised.

Homework Helper
Gold Member
Electrons have 'spin' and they are charged. That causes them to behave like small electromagnets with a Moment Dipole Moment. But the 'current' in these electromagnets cannot decay. That's a consequence of Quantum Mechanics.
In most materials, most of the time, the total of electrons with one spin equals the total with the other spin so most materials cannot be permanently magnetised.
As I understand it, only the spinning electrons at the material's surface produce the magnetic field. Interior electrons cancel each other.

Gold Member
As I understand it, only the spinning electrons at the material's surface produce the magnetic field. Interior electrons cancel each other.
Can that be correct? Where have you read that?
If you take a permanent magnet and break it half way, there will be two new poles. Also there is an internal field all the way through.

Homework Helper
Gold Member
Can that be correct? Where have you read that?
If you take a permanent magnet and break it half way, there will be two new poles. Also there is an internal field all the way through.
What you point out does not negate the idea of surface currents.
Yes there is an internal field; that's how a solenoid works ... I see no way in which my description differs in essence from a solenoid.

weirdoguy
Electrons have 'spin'

Well, yes, they have spin, but since when this means that they are spinning around their own axes?

davenn and Digcoal
Homework Helper
Gold Member
Well, yes, they have spin, but since when this means that they are spinning around their own axes?
If I may respond to @sophiecentaur 's post:
Since about 13 billion years ago, the estimate age of the universe.

Digcoal
Mentor
Several posts deleted to clean up the thread. Thanks everyone who tried to contribute positively to that.

fresh_42 and etotheipi
Gold Member
sophiecentaur
Gold Member
And so do the bands of alternating magnetisation in the widening gaps between some tectonic plates. These alternations are due to the new ferrous material coming to the surface and cooling down, aligned with the Earth's magnetic field at the time and then remaining magnetised. The magnetic stripes are evidence of when the Earth's field flips (every couple of hundred thousand years) and the records go back many cycles.

davenn
Gold Member
The permanent magnets in old earphones slowly lose power over many years. It is also normal practice to use a "keeper" on a permanent horse shoe magnet to preserve its strength. The external field of a magnet is in a direction to slowly demagnetise the magnet.

sophiecentaur
Gold Member
The permanent magnets in old earphones slowly lose power over many years. It is also normal practice to use a "keeper" on a permanent horse shoe magnet to preserve its strength. The external field of a magnet is in a direction to slowly demagnetise the magnet.
This is true but it takes us from the 'theoretical idea; to the practical reality. It's not the Field that gets 'tired' in old headphones; it's the energy levels associated with the dipole orientation in old fashioned magnetic materials. At room temperature, there is a finite probability of the occasional flip to a lower energy state. All materials can be expected to change over time - even when that change corresponds to longer than the possible life of the Universe.

The OP asks "how long will it last?". Well - how long is a piece of string?

snorkack
Iron heated to its Curie temperature of 770 degrees immediately loses all magnetism and does not recover it on cooling.
Is there any way to predict speed of magnetic creep - like, a magnet will keep half its initial strength after so much time heating at 700 Celsius, or after so much (a longer time) heating at 600 degrees, et cetera?

sophiecentaur and Dale
mitochan
The law of entropy increase in statisc or thermal mechanics would work here. The ordered spins of molecules tend to be randomized which leads zero magnetic field. The ordered motion of charges e.g. electric currents in wire would be dissipated which also leads to zero magnetic field.

sophiecentaur